test(spec): collections contract (sequences, vectors, lists, maps, sets)

Add behavioral spec suites for the collection API (~180 cases). Writing them
surfaced and fixed real bugs:
- (count nil) errored; now 0 (Clojure semantics)
- (repeat x) and (repeatedly f) — the infinite 1-arg arities — were unsupported;
  now return lazy infinite seqs
- set equality used deep= (representation-sensitive), so a set of map literals
  could differ from an equal set built differently; now value-based (each
  element value-equal to some element of the other)

Known limitation filed: phm-typed maps (from hash-map) used as set elements /
map keys hash by identity (map *literals* are structs and work).

jpm test green.

test/spec/lists-spec.janet

@@ -0,0 +1,28 @@
+# Specification: lists (persistent singly-linked).
+(use ../support/harness)
+
+(defspec "list / construct & predicate"
+  ["list"                   "[1 2 3]"   "(list 1 2 3)"]
+  ["empty list"             "[]"        "(list)"]
+  ["quoted list"            "[1 2 3]"   "(quote (1 2 3))"]
+  ["list? true"             "true"      "(list? (list 1 2))"]
+  ["list? on conj result"   "true"      "(list? (conj (list 1) 0))"]
+  ["count"                  "3"         "(count (list 1 2 3))"]
+  ["empty? true"            "true"      "(empty? (list))"]
+  ["list = vector elts"     "true"      "(= (list 1 2 3) [1 2 3])"])
+
+(defspec "list / access & update"
+  ["first"                  "1"         "(first (list 1 2 3))"]
+  ["rest"                   "[2 3]"     "(rest (list 1 2 3))"]
+  ["peek is first"          "1"         "(peek (list 1 2 3))"]
+  ["pop drops first"        "[2 3]"     "(pop (list 1 2 3))"]
+  ["conj prepends"          "[0 1 2]"   "(conj (list 1 2) 0)"]
+  ["conj many prepends"     "[4 3 1 2]" "(conj (list 1 2) 3 4)"]
+  ["cons prepends"          "[0 1 2]"   "(cons 0 (list 1 2))"]
+  ["nth"                    "20"        "(nth (list 10 20 30) 1)"])
+
+(defspec "list / immutability & performance"
+  ["conj does not mutate"   "true"      "(let [l (list 1 2 3) m (conj l 0)] (and (= l [1 2 3]) (= m [0 1 2 3])))"]
+  ["reduce conj builds"     "[2 1 0]"   "(reduce conj (list) (range 3))"]
+  ["O(1) conj at scale"     "200000"    "(count (reduce conj (list) (range 200000)))"]
+  ["scale head correct"     "199999"    "(first (reduce conj (list) (range 200000)))"])